1. Field of the Invention
The present invention basically relates to an optical amplitude modulation system. In particular, the present invention is directed to an optical amplitude modulation system which is capable of performing high extinction ratio modulation by suppressing high order components which are generated when DSB modulation is performed.
2. Description of the Related Art
In optical communication, light is modulated to have signal information. As optical modulation, direct modulation and external modulation are known. The direct modulation modulates a driving power of semiconductor laser. And the external modulation modulates light from semiconductor laser by means other than light source. A modulator used in direct modulation is generally called an optical modulator. The optical modulator modulates optical intensity, phase, etc. by causing physical changes in the optical modulator based on signals. As technical problems of the optical modulator, there exist reduction of driving voltage, realization of a higher extinction ratio for improving modulation efficiency, widening a bandwidth, and improvement of high light utilization efficiency for speeding up and loss reduction of a modulation. In other words, development of a modulator having high extinction ratio is desired. It is to be noted that the extinction ratio is a ratio of optical intensity of the highest level to the lowest level.
An optical signal side-band (optical SSB) modulator is a modulator which shifts frequency of an optical signal and outputs the optical signal [Tetsuya Kawanishi and Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IEICE, OCS2002-49, PS2002-33, OFT2002-30 (2002-08)].
An optical FSK modulator which is a modification of an optical SSB modulator is also known [Tetsuya Kawanishi and Masayuki Izutsu, “Optical FSK modulator using an integrated light wave circuit consisting of four optical phase modulator”, CPT 2004G-2, Tokyo, Japan, 14-16 Jan. 2004] [Tetsuya Kawanishi, et al. “Analysis and application of FSK/IM simultaneous modulation” Tech. Rep. of IEICE. EMD 2004-47, CPM 2004-73, OPE 2004-130, LQE 2004-45 (2004-08), pp. 41-46].
In order to remove a third order component, a following technique by means of the optical SSB modulator is known. The technique is applying a signal whose frequency is controlled and also applying an electric signal which is controlled so that third order components and optical signals are reversed in phase (see, for example, Japanese Unexamined Patent Application Publication No. 2004-77835).
On the other hand, carrier signal suppressed DSB-SC modulator is also known. This modulator generates high order components (especially, odd order components), but these components are not thought to be a substantial problem. Since the DSB-SC modulator and the optical SSB modulator have different voltage levels applied and different methods of applying the voltage, there is a problem that a method of removing the third order components which is taken by the SSB modulator cannot be applied to the DSB-SC modulator.
It is an object of the present invention to provide a new optical amplitude modulation system.
It is an object of the present invention to provide an optical amplitude modulation system capable of suppressing the third order components, thereby obtaining a high extinction ratio.